Most silicone coated release liners, which are used with labels and business-forms, have a super-calendered kraft base paper. These super-calendered sheets are very dense and prevent the silicone from soaking into the fibers before curing. The densified kraft or coated base papers curl drastically with small changes in temperature or humidity. This causes problems not only in manufacturing, but also when run through laser printers or the like at customer's installations. The calendering process also leaves a very smooth surface so that sometimes frictionizing coatings need to be applied to the liner opposite the silicone coating to aid in printer feeding when forms or labels utilizing the release liner are fed through non-impact printers, such as laser printers. The frictionizing coatings can rub off, contaminating printer parts.
While bond papers or non-densified kraft papers are known to have a more open construction more like the face sheets of the labels or business forms formed thereby, which results in less curling, it has been extremely difficult to apply a thin, smooth, continuous layer of silicone to the surface of such papers. Therefore these papers either have not been used or if used have less desirable properties than conventional release liners. One proposal for remedying this problem (see U.S. Pat. No. 5,023,138) has been to use a hot melt material having wax like properties that penetrates the porous paper substrate to provide a barrier coat so that the silicone can be provided thereover, but this increases the cost of production because it requires another step, and another coating material.
According to the present invention a release liner, a business form or label construction utilizing a release liner, and a method of producing business forms or labels using a laser printer or the like, are provided which overcome the problems set forth above. One of the key features of the present invention is to use a substrate of carbonizing bond paper for the release liner, onto which a radiation curable silicone release material is coated. Carbonizing bond paper is the paper that is a carrier for carbon in the manufacture of carbon paper. A typical range of properties of carbonizing bond paper is conventionally recognized as follows:
______________________________________ 16 lb. 20 lb. ______________________________________ Basis Weight (17 .times. 22 - 500) 15.2-16.8 19.0-21.0 (g/m.sup.2) 71.4-79.1 Caliper (mils) 3.2-3.64-2.8 Porosity, Sheffield units 5-45 (21/4" orifice) Opacity, Bausch Lomb (%) 85-93 Smoothness, Sheffield - FS 110-160 110-160 WS 60-110 Brightness - FS 75-855 WS 75-8585 Ink Penetration (Hercules) - FS 10-25 10-25 Wax Pick - FS 12-18 12-18 Ash (%) 15-32 15-18 ______________________________________ FS -- Felt Side WS -- Wire Side
Carbonizing bond, the base paper, usually is coated paper, ranging in weight from 12 lb. to 20 lb. (17.times.22-500). The pigments used in the base paper coating and/or on the coating serve to increase the opacity of the sheet, which minimizes the show-through of the carbon ink applied on the back of the paper. The coating tends to cover up any pinholes in the base paper so that the carbon ink does not strike through to the front of the paper, and it increases the release and receptivity of the carbon ink.
Carbonizing bond paper is distinct from conventional bond paper in that it provides more hold-out, which allows a conventional coater to apply a thin, smooth, continuous layer of silicone release material directly to a face of the carbonizing bond paper. However it does not have the same lack of permeability as super-calendered kraft base paper. Also, carbonizing bond paper has faces which are relatively rough, much rougher than the very smooth surfaces of calendered papers, so that there is no necessity for providing a frictionizing coating to facilitate printer feeding. The carbonizing bond paper utilized as the release liner according to the present invention preferably is lightweight, e.g. having a weight between about 35-80 grams per square meter (gsm), preferably about 45-60 gsm.
According to one aspect of the present invention a release liner for business form and label constructions having pressure sensitive adhesive is provided. The release liner comprises a substrate of carbonizing bond paper having a weight of between about 35-80 gsm (preferably about 45-60 gsm), and having first and second faces. The radiation curable silicone release material is coated directly on the first face of the substrate. That is no barrier coat is necessary, the carbonizing bond having sufficient porosity to allow some penetration of the silicone, but insufficient porosity (such as standard bond, newspaper, or like uncalendered papers have) to allow deep penetration. Therefore a thin, smooth, continuous layer of silicone release material may be coated directly on the first face.
The silicone release material preferably comprises a polymer blend having a coating weight of between about 0.8-2.3 gsm (e.g. between 1.2-1.9 gsm) applied to the first face, and the second face is bare (that is uncoated, no frictionizing coating being necessary to provide printer feeding). The coating may be a substantially continuous coating (that is over substantially the entire area to which it is applied), or it may be pattern coated. One exemplary pattern is a plurality of parallel spaced strips, which are easy to apply with conventional coaters, although spots, dots, diamond-shape, or other patterns also may be provided as long as they allow sufficient release of pressure sensitive adhesive which will contact the silicone release material.
The silicone polymer blend may comprise 30-99% base silicone polymer, 0-40% tight release additive, 0-50% easy release additive, 0-69% substantially inert ingredients, and about 1-5% photoinitiator. While both tight and easy release additives may be used, preferably only one or the other is used. For example the silicone polymer blend may comprise or consist essentially of about 85-98% base silicone polymer, about 1-10% tight or easy release additive, and about 1-5% photoinitiator; or the silicone polymer blend may consist essentially of about 95-99% base silicone polymer and 1-5% photoinitiator.
A business form or label construction (typically for use in a non-impact printer, such as a laser printer, which generates heat as part of the curing process) also is provided according to the invention. The business form or label comprises a paper face sheet (e.g. conventional bond paper) typically having a weight of between about 65-99 gsm (e.g. about 75-85 gsm) and first and second faces. In one aspect of the invention a pressure sensitive adhesive is applied to the silicone coating on the release liner. The adhesive then transfers to the second face of the face sheet when the face sheet is pressed into engagement with the adhesive layer. The adhesive transfers to the face sheet since it has greater affinity for the face sheet than the silicone coating. In another aspect of the invention a pressure sensitive adhesive coating is applied to the second face of the face sheet, e.g. a hot melt permanent or removable adhesive. The release liner comprises a substrate of carbonizing bond paper having first and second faces and a cured radiation curable silicone polymer blend coated directly on the first face of the substrate, the silicone polymer blend of the release liner engaging the pressure sensitive adhesive. The details of the carbonizing bond and the silicone material may be as set forth above.
The invention also contemplates a method of producing business forms or labels using a laser printer. The method comprises the steps of: Providing a business form or label in sheet form comprising: a paper face sheet having a weight of between about 65-99 grams per square meter, and first and second faces; a permanent or removable hot melt pressure sensitive adhesive coating said second face of said face sheet; and a release liner comprising a substrate of carbonizing bond paper having a weight between 35-80 grams per square meter, and having first and second faces; and a cured UV curable silicone polymer blend coated directly on said first face of said substrate or on the first face of the release liner; said silicone polymer blend of said release liner engaging said pressure sensitive adhesive. And, passing the sheet through the laser printer to print indicia on the paper face sheet first face, substantially without curl of the business form or label. The providing step may be further practiced by providing the particular silicone polymer blend as set forth above.
The release liner, and the business form or label and method utilizing the release liner, according to the present invention will thus be seen to have most of the advantages of the prior art with few of the drawbacks. The release liner according to the present invention is relatively inexpensive, does not require a barrier coat before application of the silicone release material, yet the silicone release material may be readily applied in a thin, smooth continuous layer using conventional equipment, and the release liner face opposite the silicone release material has no need for a frictionizing coating. The release liner is easy to manufacture, not being nearly as susceptible to small changes in temperature or humidity as conventional supercalendered kraft papers coated with silicone which are used as release liners, and when used in laser or like printers have much less a tendency to curl than conventional super-calendered kraft paper release liners.